Effect of Monensin and Protein Supplementation on Intake and Ruminal Fermentation Parameters in Cattle Consuming Low-Quality Forage.

The effect of monensin (Rumensin® 90) and protein supplementation on intake, digestion, and ruminal fermentation parameters on cows consuming low-quality forage (LQF; 4.1% CP chopped bluestem hay) was evaluated. Four ruminally-cannulated cows (637 ± 24 kg BW) were used in a 4x4 Latin Square design (20 d periods). Treatments were arranged as a 2x2 factorial: first factor was monensin (0 or 200 mg•hd^-1•d^-1) and second factor was protein supplement (0 or 0.64 kg•hd^-1•d^-1 CP provided as cottonseed meal). A premix (0.23 kg∙hd^-1∙d^-1) consisting of ground hay, cracked corn, molasses, salt, dicalcium phosphate, and a commercial mineral premix, was provided to all animals and allowed for inclusion of monensin. Animals were housed and fed individually at 0600 h daily. To prevent carry over effects from previous monensin feeding, 14 days were required before sampling. However, to obtain an optimal response from monensin, only 10 days were needed for treatment adaptation. Thus, during d 1-4 of each period all animals were fed LQF with no treatment. Day 5-14 allowed for treatment adaptation and day 15-20 for sample collection. Intake measurements were taken d 15-19. Rumen fluid was collected h 0, 2, 4, 8, 12, 16, and 20 on d 20 to determine pH. Data were analyzed using the MIXED procedure in SAS 9.3 (SAS Inst. Inc., Cary, NC) with terms in the model including protein, monensin, and their interaction, with animal and period included as random effects. No protein × monensin interaction (P = 0.37) or effect of monensin (P = 0.32) was observed for any measure of intake. An effect of protein was observed for total OM intake (TOMI; P < 0.01), forage OM intake (FOMI; P < 0.01), total NDF intake (TNDFI; P < 0.01), and forage NDF intake (FNDFI; P < 0.01). Protein increased TOMI by 88.4%, FOMI by 86.8%, TNDFI by 61.6% and FNDFI by 55.2%. A protein × monensin × hour interaction was observed (P = 0.005) for pH. This interaction was likely caused by the interaction of protein with hour after feeding and the interaction of protein and monensin at 0 h after feeding. Results suggest that adding monensin to a protein supplement for cattle consuming LQF will not provide an added improvement on intake compared with protein alone.